1. Field of the Invention
The present invention relates to a process for preparing 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMP-N-oxyl) and its 4-position substituted derivatives by oxidizing 2,2,6,6-tetramethylpiperidine (TEMP) or its 4-position substituted derivatives with hydrogen peroxide in the presence of low concentrations of water-soluble light or heavy divalent metal salts: ##STR1## wherein formula 2 above is TEMP and formula 1 above is TEMP-N-oxyl and R.sub.1 is hydrogen, a linear or branched C.sub.1 -C.sub.20 alkyl group (particularly methyl, ethyl, n-and i-propyl and n- and tert-butyl), benzyl, ##STR2## and R.sub.2 is hydrogen, a linear or branched C.sub.1 -C.sub.20 alkyl group or phenyl, and MeY is the divalent metal salt.
2. Description of the Prior Art
The synthesis of N-oxyls by oxidation of tertiary amines is known in the literature and differ from the present invention primarily with respect to the oxidizing agent used. For Example, R. Winter and R. Malherbe conduct the oxidation of N-oxyls with organic hydroperoxides, e.g., tert-butyl hydroperoxide (see EP-A 0 157 738), thus using an expensive oxidizing agent which produces a coupling byproduct in at least stoichiometric quantities. This byproduct also exists when percarboxylic acids are used as oxidizing agents. Chou et al use, for example, 3-chloroperoxide benzoic acid as an oxidizing agent (see J. Org. Chem. 39 [1947] 2356, 2360).
A much better oxidizing agent than organic hydroperoxides and percarboxylic acids is hydrogen peroxide because it is inexpensive and because water is produced as the coupling byproduct. D. P. Young et al use hydrogen peroxide as an oxidizing agent in the presence of a salt of tungstic acid as catalyst (see GB-P 1 199 351 and Tetrahedron 20 [1964] 131, 137). The drawbacks of this process, however, are the extremely long reaction times (on the order of several days) and the problem that arises when disposing of the catalyst, since it may not be flushed away with the waste water for reasons relating to environmental regulations, etc. Even when sodium carbonate is used, long reaction times result (see Soviet Physics Doklady 261, 1, 103-110, [1981]).
Tungstophosphoric acid has been used to reduce the reaction time of oxidation, but again the problems relating to disposal of the catalyst and its expense remain (see Cf. R. Briere, H. Lemaire and A. Rassat, Bull. Soc. Chim. France, 11, 3273 [1965]).
In addition to these problems, all the above-described oxidation methods have a high rate of consumption of chemical oxidizing agents and catalysts, and employ complicated procedures, while some are very time-consuming.